There is available an image printing apparatus which forms a latent image on a photosensitive body while scanning the photosensitive body using a beam, develops the latent image using toner, and transfers the image onto a recording medium. Examples of such image printing apparatus include a multibeam image printing apparatus which forms a latent image using a plurality of beams.
Assume that in a general multibeam image printing apparatus, all beams have the same image transfer clock and same synchronous detection signal. In this case, since the beams have different optical path lengths, an image as schematically shown in FIG. 13 is obtained as a latent image to be formed or an image developed and transferred onto a recording medium. This means that a plurality of beams have different main scanning widths.
To adjust a difference in main scanning width between beams, there is available a method of forming test patterns on a photosensitive body by the beams, detecting the positions of the patterns by a sensor, and adjusting the image transfer clocks and scanning start positions on the basis of the positions. The test pattern formation, position detection, and position-based beam scanning position (irradiation position) adjustment are executed for each of the plurality of beams.
As prior-art references that pertain to an image printing apparatus using a plurality of beams, there are Japanese Patent Laid-Open Nos. 2001-013430 and 2001-071554. Japanese Patent Laid-Open No. 2001-013430 discloses an arrangement which executes four-line scanning. In the arrangement, a time period between a synchronous detection signal for each line detected by a photodetector and a synchronous detection signal at a trailing edge of each line detected by the photodetector is counted, the image transfer clocks for the four lines are controlled on the basis of the count values such that the magnifications of the respective beams on the photosensitive body are constant and equal.
In a method of forming test patterns for all of a plurality of beams and adjusting beam scanning positions, the amount of toner used increases in proportion to the number of beams, and test pattern formation and test pattern position detection need to be repeated a number of times equal in number to the beams. For example, a four-beam, four-color multibeam printer needs scanning position adjustment for 16 beams in total. This printer requires toner enough to develop test patterns for the 16 beams. Since test pattern formation and test pattern position detection are executed for each of the 16 beams, the calibration time becomes longer.